The present invention relates to high temperature superconductors, more particularly to the use thereof in filters that may be suitable for electronic applications such as those involving communications or radar.
At the front end of practically every antenna (e.g., microwave or radio frequency receiver antenna) is a filter that eliminates (cuts off or excises) all frequencies outside of a predetermined frequency window (sometimes referred to as a “bandpass” or “bypass band”), thereby preventing the totality of the environmental signals from overwhelming the device. The operational principle of a typical filter is similar to that of a typical resonator cavity, which is designed to resonate at a predetermined frequency window (sometimes referred to as the resonator's “resonance frequency”) where transmission is at its maximum, while at all other frequencies (i.e., frequencies outside of the resonance frequency) transmission is strongly suppressed.
A “flat” (frequency-independent) resonance frequency window is typically obtained from the combined effects of a series of inductively coupled narrow copper strips, which may be arranged in a variety of configurations. Each strip gives rise to a pole in the transfer function of the configuration; hence, the terms “strip” and “pole” have been used interchangeably in filter technology. The ensuing box-like bandpass is, roughly speaking, the sum of the slightly shifted hump-shaped (e.g., Gaussian-shaped, Chebyshev-shaped, Lorentzian-shaped, etc.) frequency windows that are each associated with a particular pole. A “linear” material is a material in which microwave transmission does not depend on the field intensity. For a filter made of a linear material, as the number of poles increases the bandpass approaches the ideal box-like shape. However, an increase in the number of strips, in combination with surface-impedance nonlinearity (the existence of which depends on the material) in each strip, represents the cause for the generation of intermodulation distortion (IMD) products. Surface-impedence nonlinearity is a property of high temperature superconductor (HTS) materials.
The term “intermodulation distortion” (“IMD”) refers to the undesirable mixing of two signals whose mixing products lie within the bandpass. A case in point is the mixing of signals lying outside the nominal bandpass with signals lying within the bandpass, thus producing added frequency components that contribute to distortion of the desired signals. IMD arises as a consequence of surface-impedence nonlinearity and perhaps other sources.
The IMD power level is a key performance measure of a filter. Copper-based filters are commonly used for antenna applications. In copper-based filters, where copper is highly linear, increasing the number of poles in order to approach a box-like frequency window constitutes a trade-off between an increase in the physical size on the one hand and losses of the device on the other hand. It would be desirable to provide a filter having all three attributes, viz., low IMD power level, low loss, and small physical size. The combination of these qualities in a filter could unleash new opportunities for various applications, both military and civilian, such as involving antennae arrays for radar applications and specialized (e.g., compact and sensitive) antennae aboard missiles and submarines. Generally speaking, HTS-based filters have two of these qualities, viz., extremely low losses and compactness, but are also characterized by surface-impedence nonlinearity and hence by tendency toward high IMD power levels.
The high temperature superconductor (HTS) family of materials has seen commercial success in the area of microwave filters for wireless communication. Over fifteen hundred HTS microwave filter units have been deployed in wireless communication base-stations; see R. W. Simon, R. B. Hammond, S. J. Berkowitz and B. A. Willemsen, Proceedings of the IEEE 92, 1585 (2004), incorporated herein by reference. In such applications, the copper poles are replaced with HTS poles. The commercial success of HTS microwave filters is mainly attributable to their practical and cost-effective cooling requirements (to T=77K, the liquid Nitrogen temperature), their relatively small size, and the much lower losses of HTS in comparison to those of copper (by two to three orders of magnitude at microwave frequencies). However, because of the surface-impedance nonlinearity that characterizes HTS, progress in this area is limited to low power applications. See the following publications, each of which is incorporated herein by reference: J. H. Claasen, J. C. Booth, J. A. Beall, L. R. Vale, D. A. Rudman and R. H. Ono, Superconductor Science and Technology 12, 714 (1999); J, C, Booth, L. R. Vale, R. H. Ono and J. H. Claasen, Superconductor Science and Technology 12, 711 (1999); H. Claasen, J. C. Booth, J. A. Beall, D. A. Rudman, L. R. Vale and R. H. Ono, Applied Physics Letters 74, 4023 (1999); J. C. Booth, J. A. Beall, D. A. Rudman, L. R. Vale and R. H. Ono, Journal of Applied Physics 86, 1020 (1999). IMD suppression is increasingly critical for operation in the increasingly crowded cellular phones communication spectrum. In addition, HTS nonlinearity must be reduced to realize emit-filter applications. HTS nonlinearity at microwave frequencies thus represents a bottleneck issue for future HTS filter applications.